<PAGE>

                                                                   EXHIBIT 10.21

                                                           [LETTERHEAD OF SIPEX]

September 11, 2002

Joe Rauschmayer
13877 Lyade Ave.
Saratoga, CA 95070

Dear Joe:

It is a pleasure to extend an offer of employment to join Sipex Corporation in
the position of Senior Vice President of Operations, reporting to Walid
Maghribi, CEO. Your total base compensation package would be $4,423.07/week
($230.000.00 annualized). In accordance with our compensation plan, your
performance and salary will be reviewed on an annual basis. We will recommend to
the Board of Directors that you be issued 300,000 stock options at the fair
market value at the time of issuance. These options will be issued in accordance
with our Stock Option Policy. In addition, Sipex has previously had in place a
Management Incentive Plan. These plans were suspended as a direct result of the
recent economic downturn in the semi-conductor market. However, should those
plans be reinstated your position will be eligible for inclusion. This offer is
contingent upon the successful completion of a satisfactory background check.

If your employment with the Company terminates other than voluntarily or for
Cause, and you sign and do not revoke a standard release of claims with the
Company, then you shall be entitled to (i) receive continuing payments of
severance pay (less applicable withholding taxes) at a rate equal to your Base
Salary rate, as then In effect, for a period of six (6) months from the date of
such termination, to be paid periodically in accordance with the Company's
normal payroll policies: and (ii) 50% of any unvested shares of any option shall
immediately vest and become exercisable until the earlier of (i) the original
term of the option or (ii) twelve (12) months from the date of termination.

If your employment with the Company terminates voluntarily by you or for Cause
by the Company, then (i) all vesting of the Option will terminate immediately
and all payments of compensation by the Company to you hereunder will terminate
immediately (except as to amounts already earned), and (ii) you will only be
eligible for severance benefits in accordance with the Company's established
policies as then in effect.

Notwithstanding the foregoing, in the event your employment with the Company
terminates voluntarily by you or other than for Cause by the Company in
connection with a "Change of Control" (as defined herein) that occurs within one
(1) year of the Effective Date, you shall be entitled to (i) receive continuing
payments (less applicable withholding taxes) at rate equal to your Base Salary
rate, as than in effect, for a period of six (6) months from the date of
termination, to be paid periodically in accordance with the Company's normal
payroll policies; and (ii) 25% of the shares subject to the Option shall vest
and become exercisable. In the event your employment with the Company terminates
voluntarily by you or other than for Cause by the Company in connection with a
Change of Control that occurs more than one (1) year after the Effective Date
but within three (3) years of the Effective Date, you shall be entitled to (i)
receive continuing payments (less applicable withholding taxes) at a rate equal
to your Base Salary rate, as then in effect, for a period of six (6) months from
the date of termination, to be paid periodically in accordance with the
Company's normal payroll policies; and (ii) 100% of any unvested shares of any
option shall vest and become exercisable until the earlier of (i) the original
term of the option or (ii) twelve (12) months from the date of termination. In
the event your employment with the Company terminates voluntarily by you or
other than for Cause by the Company in connection with a Change of Control that
occurs more than three (3) years from the Effective Date, 100% of the any shares
of any option shall vest and become exercisable until the earlier of (i) the
original term of the Option or (ii) twelve (12) months from the date of
termination.

For purposes of this letter, "Cause" is defined as (i) an act of material
dishonesty made by you in connection with your responsibilities as an employee,
(ii) your conviction of, or plea of nolo contendere to, a felony, (iii) your
gross misconduct, or (iv) your continued substantial violation of your
employment duties after you have received a written demand for performance from
the Company which specifically sets forth the factual basis for the Company's
belief that you have not substantially performed your duties.

Sipex Corporation - 22 Linnell Circle - Billerica, MA 01821 - TEL: 978.667.8700
                              - FAX: 978.667.8310

    233 South Hillview Drive - Milpitas, CA 95035 - TEL: 408.934.7500 - FAX:
                                  408.935.7600

<PAGE>

For purposes of this letter. "Change of Control" of the Company is defined as:
(i) any "person" (as such term is used in Sections 13(d) and 14(d) of the
Securities Exchange Act of 1934, as amended) is or becomes the "beneficial
owner" (as defined in Rule 13d-3 under said Act), directly or indirectly, of
securities of the Company representing sixty percent (60%) or more of the total
voting power represented by the Company's then outstanding voting securities; or
(ii) a change in the composition of the Board occurring within a two-year
period, as a result of which fewer than a majority of the directors are
Incumbent Directors. "Incumbent Directors" will mean directors who either (A)
are directors of the Company as of the date hereof, or (B) are elected, or
nominated for election, to the Board with the affirmative votes of at least a
majority of the Incumbent Directors at the time of such election or nomination
(but will not include an individual whose election or nomination is in
connection with an actual or threatened proxy contest relating to the election
of directors to the Company); or (iii) the date of the consummation of a merger
of consolidation of the Company with any other corporation that has been
approved by the stockholders of the Company, other than a merger or
consolidation which would result in the voting securities of the Company
outstanding immediately prior thereto continuing to represent (either by
remaining outstanding or by being converted into voting securities of the
surviving entity) more than sixty percent (60%) of the total voting power
represented by the voting securities of the Company or such surviving entity
outstanding immediately after such merger or consolidation, or the stockholders
of the Company approve a plan of complete liquidation of the Company; or (iv)
the date of the consummation of the sale or disposition by the Company of all or
substantially all the Company's assets. Notwithstanding the foregoing, a "Change
of Control" shall not include any transaction or series of transactions
involving the Company's issuance of any equity or debt securities to third
parties for capital raising purposes.

Employment at Sipex Corporation will provide you with the opportunity to
participate in the employee fringe benefit program which includes your choice of
a medical and dental insurance program, term life insurance, disability income,
a tax deferred investment program (401K), Employee Stock Purchase Plan and paid
holiday and vacation plans. A Benefits Choices booklet is enclosed which
summarizes your benefits. Complete details on the plans will be available when
you sign up for benefits on your first day of employment. If I can provide you
with further information regarding the position or the fringe benefits, please
feel free to contact me.

We are confident of the valuable contribution you will make toward the future
success of Sipex Corporation and we look forward to your joining us as soon as
possible. Please sign, date and return the original of the enclosed letter
confirming your concurrence with the terms of our offer as outlined above, and
indicating your firm start date. This offer is effective for three days from the
date of this letter. On your first day of work, please bring with you evidence
of your U.S. citizenship or proof of your legal right to live and work in this
country. We are required by federal law to examine documentation of your
employment eligibility within three days after you begin work.

This letter is not an employment agreement and does not constitute a guarantee
or promise of employment, it being your and our agreement, that you shall, at
all times, remain an employee at will and that your employment may be terminated
at any time by either the Company or you.

This letter (a duplicate original is enclosed for your records) along with any
agreements relating to proprietary rights between you and Sipex Corporation sets
forth the terms of your employment with Sipex Corporation and supersedes any
prior representations or agreements, whether written or oral. This letter may
not be modified or amended except by written agreement, signed by you and Sipex
Corporation.

Sincerely,

/s/ James G. Chalmers
James G. Chalmers
Human Resource Director

I understand and accept the terms of this employment offer.

/s/ Joe Rauschmayer                                    9/30/02
--------------------                            ---------------------
Joe Rauschmayer                                 Start Date

<PAGE>

                                    Addendum to: Rauschmayer Offer dated 9/12/02

September 17, 2002

Joe Rauschmayer
13877 Lynde Ave.
Saratoga, CA 95070

Dear Joe:

Pursuant to your recent conversation with Walid Maghribi, CEO and a subsequent
action by Sipex's Board of Directors, I am pleased to inform you that our offer
of employment see attached, has been amended as follows:

         Stock Option granted increased from 300,000 to 350,000 shares.

If you should have questions or comment regarding this action please call me at
(978) 671-4626.

Regards,

James G. Chalmers
Director, Human Resources
Sipex Corporation

Enclosure (1)

maa

cc: Walid Maghribi

<PAGE>

                                                           [LETTERHEAD OF SIPEX]

September 11, 2002

Joe Rauschmayer
13877 Lynde Ave.
Saratoga, CA 95070

Dear Joe:

The following letter represents an addendum to the offer dated September 11,
2002 made to you by Sipex Corporation. Your acceptance of said offer was
contingent on this addendum.

If your employment with the Company terminates other than voluntarily or for
Cause, and you sign and do not revoke a standard release of claims with the
Company, then you shall be entitled to (i) receive continuing payments of
severance pay (less applicable withholding taxes) at a rate equal to your Base
Salary rate, as then in effect, for a period of six (6) months from the date of
such termination, to be paid periodically in accordance with the Company's
normal payroll policies; and (ii) 50% of any unvested shares of any option shall
immediately vest and become exercisable until the earlier of (i) the original
term of the option or (ii) twelve (12) months from the date of termination.

If you employment with the Company terminates voluntarily by you or for Cause by
the Company, then (i) all vesting of the Option will terminate immediately and
all payments of compensation by the Company to you hereunder will terminate
immediately (except as to amounts already earned), and (ii) you will only be
eligible for severance benefits in accordance with the Company's established
policies as then in effect.

Notwithstanding the foregoing, in the event your employment with the Company
terminates voluntarily by you or other than for Cause by the Company in
connection with a "Change of Control" (as defined herein) that occurs within one
(1) year of the Effective Date, you shall be entitled to (i) receive continuing
payments (less applicable withholding taxes) at a rate equal to your Base Salary
rate, as then in effect, for a period of six (6) months from the date of
termination, to be paid periodically in accordance with the Company's normal
payroll policies; and (ii) 25% of the shares subject to the Option shall vest
and become exercisable. In the event your employment with the Company terminates
voluntarily by you or other than for Cause by the Company in connection with a
Change of Control that occurs more than one (1) year after the Effective Date
but within three (3) years of the Effective Date, you shall be entitled to (i)
receive continuing payments (less applicable withholding taxes) at a rate equal
to your Base Salary rate, as then in effect, for a period of six (6) months
from the date of termination, to be paid periodically in accordance with the
Company's normal payroll policies; and (ii) 100% of any unvested shares of any
option shall vest and become exercisable until the earlier of (i) the original
term of the option or (ii) twelve (12) months from the date of termination. In
the event your employment with the Company terminates

 Sipex Corporation - 22 Linnell Circle - Billerica, MA 01821 - TEL: 978.667.8700
                              - FAX: 978.667.8310

    233 South Hillview Drive - Milpitas, CA 95035 - TEL: 408.934.7500 - FAX:
                                  408.935.7600

<PAGE>

voluntarily by you or other than for Cause by the Company in connection with a
Change of Control that occurs more than three (3) years from the Effective Date,
100% of the any shares of any option shall vest and become exercisable until the
earlier of (i) the original term of the Option or (ii) twelve (12) months from
the date of termination.

For purposes of this letter, "Cause" is defined as (i) an act of material
dishonesty made by you in connection with your responsibilities as an employee,
(ii) your conviction of, or plea of nolo contendere to, a felony, (iii) your
gross misconduct, or (iv) your continued substantial violation of your
employment duties after you have received a written demand for performance from
the Company which specifically sets forth the factual basis for the Company's
belief that you have not substantially performed your duties.

For purposes of this letter, "Change of Control" of the Company is defined as:
(i) any "person" (as such term is used in Sections 13(d) and 14(d) of the
Securities Exchange Act of 1934, as amended) is or becomes the "beneficial
owner" (as defined in Rule 13d-3 under said Act), directly or indirectly, of
securities of the Company representing sixty percent (60%) or more of the total
voting power represented by the Company's then outstanding voting securities; or
(ii) a change in the composition of the Board occurring within a two-year
period, as a result of which fewer than a majority of the directors are
Incumbent Directors. "Incumbent Directors" will mean directors who either (A)
are directors of the Company as of the date hereof, or (B) are elected, or
nominated for election, to the Board with the affirmative votes of at least a
majority of the Incumbent Directors at the time of such election or nomination
(but will not include an individual whose election or nomination is in
connection with an actual or threatened proxy contest relating to the election
of directors to the Company); or (iii) the date of the consummation of a merger
or consolidation of the Company with any other corporation that has been
approved by the stockholders of the Company, other than a merger or
consolidation which would result in the voting securities of the Company
outstanding immediately prior thereto continuing to represent (either by
remaining outstanding or by being converted into voting securities of the
surviving entity) more than sixty percent (60%) of the total voting power
represented by the voting securities of the Company or such surviving entity
outstanding immediately after such merger or consolidation, or the stockholders
of the Company approve a plan of complete liquidation of the Company; or (iv)
the date of the consummation of the sale or disposition by the Company of all or
substantially all the Company's assets. Notwithstanding the foregoing, a "Change
of Control" shall not include any transaction or series of transactions
involving the Company's issuance of any equity or debt securities to third
parties for capital raising purposes.

SIPEX CORPORATION                              JOE RAUSCHMAYER

By: /s/ James G. Chalmess
    -------------------------                  ________________________
                                               Signature
Name: James G. Chalmess                        ________________________
                                               Date
Title: DIRECTOT, H.R